1. Field of the Invention
The present invention is directed to angle-sensing mechanisms for electric motors and, in particular, for synchronous machines.
2. Description of Related Art
Electric motors are used for a multiplicity of applications; depending on the particular use, not only the speed itself, but also the rotor position being relevant. Sensing the rotor position is particularly relevant when switching on electric machines since, relative to the stator's magnetic field, the rotor position directly and significantly influences the absolute value and sign of the torque generated. Sensing the correct angle of the rotor position is particularly relevant for the operation of synchronous machines where essentially no difference in speed (“slip”) is provided between the rotor and the stator. Under certain circumstances, a deviation of only a few degrees can result in a very pronounced difference with respect to the torque. In synchronous machines, a peripheral magnetic field is generated by a multiplicity of mutually offset stator windings, the rotor having a permanently impressed direction of magnetization. In principle, the magnetic field of the rotor can be generated by a permanent magnet, an excitation winding or by a combination thereof.
It is known to use separate sensors, for example, inductive or optical sensors, for the purpose of angle sensing. The use of separate sensors entails additional space, additional costs, separate supply leads, and additional manufacturing steps, etc. Moreover, additional components have the inherent risk of generally being associated with additional sources of error.
Therefore, methods and devices are known for detecting the rotor position of a motor via the windings and the rotor, without the need for additional sensors.
The German patent publication DE 10 2006 004 166 describes a method for determining the rotor position of an electronically commutated motor, the different saturation limits of the rotor in various rotor directions being used to detect the rotor position. To this end, rise times up to defined limit values are recorded, from which it is possible to measure the saturation of the rotor portion covered by the magnetic field that is generated by a stator winding corresponding to the direction. The position of the rotor is sensed from the rise in the phase currents of the stator, since the rotor exhibits an angle-dependent saturation. On the one hand, the method is costly since the angular position is based on the combination of a multiplicity of various rise times that are indicative of the particular saturation, and, on the other hand, since the resolution is defined by the angular offset of two different stator windings, i.e., 60°, for example. Thus, this publication only provides for a correlation to specific angular sectors, but not for a more precise angle sensing.
The Austrian patent publication AT 397727 B describes a method that utilizes voltage pulses to determine the rotor angle. It provides for assigning inductances, which are derived from current pulses and corresponding induced voltages, to a sinusoidal characteristic of the inductance along the periphery, in order to determine the actual instantaneous position within the sinusoidal curve from the ascertained inductance values and the derivations thereof. A complex interpolation is required to assign the measured values to the assumed sinusoidal induction curve, so that rigorous demands are made of a measured-data processing unit which interpolates the voltage and current pulses to the sinusoidal curve. In addition, inaccuracies in the position can result at locations where the inductance only changes slightly with the angle, i.e., at +/−90°, for example.
Therefore, the absolute angle sensing methods mentioned above in the context of stationary electric motors are associated, on the one hand, with additional components, or they entail a costly calculation and, at the same time, are prone to a reduced accuracy and a high fault rate due to external stray fields. In particular, interpolation-based angle acquisitions require a considerable outlay for complex calculations; even in the case of minor measurement errors, the interpolation also leading to substantial angular errors.